This invention is generally directed toward systems for bringing a spacecraft to a desired attitude. A position-based gyroless control has been invented to perform the acquisition maneuver for the spacecraft with no gyro information available but with position sensors (e.g., star trackers), which could be in a long intrusion during the maneuver.
The systems disclosed herein enable gyroless spacecraft acquisition maneuvers including power acquisitions with the spacecraft in either stowed or deployed configurations, wing/reflector deployments, on-station large-angle slew, antenna mapping, attitude hold in momentum dump, etc.
In a typical spacecraft attitude control system (ACS), a gyro plays a crucial role in performing the spacecraft acquisition maneuvers, in either stowed or fully deployed configurations. However, due to limited resources, a program may opt to use only a single on-board gyro, which drastically increases the risk factor in the area of fault autonomy. As other position sensors, such as star trackers, become more advanced and powerful, an algorithm using attitude reference, instead of rate reference, is needed for the spacecraft with no inertial reference unit (IRU) data available in either the toggle alternate or post-toggle state.
Disclosed herein is a position-based gyroless control system using position sensors to drive the spacecraft acquisition maneuvers under various maneuver scenarios. Furthermore, a sensor intrusion will prevent an attitude estimate update when a position sensor is used, resulting in an attitude drift from intrusion. A method is provided to lessen the impact of sensor intrusions using the position control technique disclosed herein.
In the past, gyroless operation existed but was only applicable for a few minutes during the entire mission on a need basis, i.e., to switch the gyro to its backup assembly when the primary unit failed. Before the redundant gyro warmed up, the ACS needed a “gyroless” control to monitor the system stability for a short while. For such a momentary gyroless ACS, analysts in the past simply relied on the so-called “derived rate” from the position sensors such as star trackers or earth sensors. They used the existing control laws to close the ACS loop, as if the rate information was always available. Some prior spacecraft have turned off the gyro during the on-station safe-hold mode, but they could not (did not) perform any gyroless “maneuvers” as described herein.
“Deriving the rate” when the gyro is not available tends to produce huge noise and is much worse than the original rate information. One can filter the noise to some extent but at a cost of sacrificing system stability margins. As a result, this noisy “artificial” rate information can produce poor performance, saturate the control actuation capability, limit the controllability and observability of the ACS, and eventually jeopardize the entire spacecraft attitude stability and cause the spacecraft to tumble. In the past, since “gyroless” operation only happened less than 0.001% of the mission time span, using a “derived rate” temporarily was acceptable before rate noise drift contaminated the ACS. But for a serious official “gyroless” mission maneuver, a “derived rate” becomes awkward and troublesome and an algorithm using the position sensors is critically needed.